Yggdrasil is the "World Tree", an immense mythical tree that was the center of the cosmos and connected the nine worlds in Norse mythology: Gimle, the High Heaven; Asgard, the home of the gods; Midgard, middle earth and home to mortals; Vana Heim, World of the Vanirgods; Alf Heim, World of the Elves; Jotun Heim, World of Giants and Trolls; Darkalf Heim, World of the Gnomes, Hel/Nifl Heim, Underworld; and Musdel Heim, World of Fire. The branches of Yggdrasil extended far into the heavens, and the tree is supported by three roots that extend far away into other worlds. Creatures live within Yggdrasil, including the dragon of destruction Níðhöggr in the roots and an unnamed eagle in the branches. A squirrel called Ratatoskr scurries up and down the tree Yggdrasil carrying insults between the eagle and Níðhöggr.
Eventually, the branches and leaves of Yggdrasil began to wither and droop, signaling the coming of Ragnorokk, the day of reckoning when the destiny of the god would be decided. Demons rose from Hel and a battle raged between the gods and all other beings. Níðhöggr, the dragon of destruction, finally chewed through the roots of Yggdrasil and it broke, and the sacred halls of Asgard toppled and fell.